As technology is demanding the construction of ever smaller structures in electronic, optical, and micromechanical systems, defects on the order of nanometers or tens of nanometers can adversely affect the performance of devices. Such defects are routinely examined using electron microscopes to determine and correct the cause of the defects. Defects can include contaminant particles that become embedded in a product during fabrication or a manufacturing defect, such as a bridge creating a short circuit between two closely spaced conductors that are intended to be electrically separated from each other.
Dual beam instruments that include a focused ion beam column and an electron beam column, such as the CLM™ system manufactured by FEI Company, the assignee of the present invention, are used in the analysis of defects and process monitoring. Defects are typically located using a whole wafer scanning inspection system, such as those manufactured by KLA-Tencor. Those systems generate a defect file that includes the coordinates of all defects found. Defects are typically found by comparing one portion of a fabricated wafer with other portions of the wafer that are intended to be identical. Deviations from the normal appearance indicate the presence of a defect. A defect may be on the surface of the wafer or below the surface, covered by subsequently applied. While such inspection systems can locate defects, they are typically unable to provide detailed information about the defect, and so the wafer is typically then loaded into an imaging instrument, such as an electron microscope, to characterize the defect in more detail.
In dual beam instruments that are intended to monitor a fabrication process, such as a photolithography process, image recognition software finds features of interest and automatically measures the features of interest. The precise location of the features is known from the design data for the lithography mask or from the wafer layout data. The focused ion beam system can then mill a trench in the substrate to expose a cross section at the desired location for observation.
It is more difficult, however, to observe defects because their position is not known with the same precision. The defect positions determined by the scanning instruments are accurate only to within a few microns. Also, the coordinate systems used by the scanning device and the coordinate system of the imaging system are typically offset because of differences in the positioning of wafer in each system. The differences in the coordinate systems can be compensated by using an offset determined by measuring the same reference points in both systems. Even with a calculated offset, the defect position is still only accurate to a few microns. Because defects as small as tens of nanometers can adversely affect the functioning of products, it is necessary to locate and examine such small defects in detail.
One method of finding and examining a defect is using the Slice-and-View™ technique that is incorporated into the software of the Defect Analyzer™ 300 from FEI Company. In the Slice-and-View technique, the general area of the defect is first located from the coordinates in a defect file from an inspection instrument. A thin, preferably conductive, protective layer is deposited in the area around the defect location, for example, by using charged-particle-beam induced deposition from a precursor gas. For example, a tungsten layer may be deposited using an ion or electron beam to decompose a precursor gas, such as tungsten hexacarbonyl. After the defect area is located, a reference mark, referred to as a fiducial, such as an “X”, is milled with the focused ion beam to provide a reference point and orientation near the defect site.
A sloping trench is then milled to expose a cross section in front of the expected defect location. An image of the cross section is then formed using the electron beam. Additional material is then milled from the cross section face to expose a new cross section face about typically about 0.03-0.04 microns from the first face, although the second cut can be made up to a micron or more from the cross-section face, depending on the size of the defect. A second electron beam image is then formed after the cut. The process of removing a small amount of material form the cross section face and forming an image is repeated, typically fifteen or twenty times. As the cross section wall progresses through the expected defect location, it is probable that one or more of the cuts will provide a useful image of the defect. The Slice-and-View technique is required, in part, because the position of the defect is not known with sufficient precision to directly position the beam to cut and observe a cross section of the defect. The Slice-and-View technique an also provide three dimensional information about some defects.
Even when an image of the defect is captured, it may be difficult to identify the defect or the separate processing layers around the defect, because there may be no or limited contrast between similar materials in a charged-particle-beam image. Electron or ion microscopy provides an image by detecting secondary electrons that are emitted when the primary beam of electrons or ions impacts the sample surface. The number of secondary electrons emitted and detected for each impacting primary beam particle depends on the composition and the topography of the sample. The electron beam image clearly shows topography and interfaces between certain materials, such as the interface between a metal layer and an oxide layer. The electron beam image does not, however, show very clearly interfaces between different materials that have similar secondary electron emission properties. For example, an electron beam image may not clearly show the boundary between different dielectric layers, such as an oxide layer and a nitride layer. To observe the interface between such layers, a process known as “decoration” is used. Decorating a cross section entails lightly etching the cross section using an etchant that etches the two materials at a different rate. The different etch rates leave a topographical feature at the interface of the two layers, and the topographical feature, such as a step, is then readily observable in the electron beam image.
Another method of improving contrast is to apply a material that preferentially binds to different materials and thereby changes the secondary electron emission characteristics of those materials. For example, the contrast between different polymers and between some biological materials can be improved using a stain of a heavy metal salt. U.S. patent application Ser. No. 11/893,022 for “Method of Obtaining Images from Slices of a Specimen,” which is assigned to the assignee of the invention and which is hereby incorporated by reference, describes the use of a Slice-and-View technique that employs staining using, for example, osmium tetroxide (OsO4). The stain preferentially binds to materials on the sample surface and does not remove any material from the substrate.
Unlike staining, the decoration process etches away material. It has not been possible to use decoration with a Slice-and-View technique because both processes remove material, and yet each slice must be sufficiently thin to ensure that a very small defect is captured in a cross section for viewing and not entire removed with a slice between observations.
Another complicating factor is that as the slices become thinner, the tungsten protective layer deposited on the surface has an increased tendency during milling to redeposit on the walls of the cross section, thereby creating artifacts and obscuring details of the cross section. Thin slices also have a tendency to “punch through” the cross section wall and mill below the wall.